CGMiner with one block erupter having 0 Accepted Shares

Hi,

I'm still new to mining. I have a Raspberry Pi with a 7 port USB Hub, 5VDC 4A AC Adapter, and 7 block erupters. That should be sufficient power. I am running the latest cgminer v.3.8.1. All 7 erupters are averaging 335MH/s, but the 7th has 0 Accepted Shares. I have a fan running off of the second usb port on the Pi, and tried swapping locations with the 7th erupters, but the result was the same. CGMiner shows it hashing on the miner.php page, and the device shows up when running lsusb, but none of the work is accepted. How do I get this last erupter's work accepted?

I'm not running cgminer with any particular settings beyond those for the pool, with the url, user, and pass values. My suspicion is power, based on what I'm reading, but the AC Adapter's 20W should be sufficient, and cgminer reports that it's averaging 335MH/s along with the rest. Thanks for any help in advance!